Spectral Cycles - what would it need to be accepted?

Contributing to Blender
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As in the title, what would the acceptance criteria be for an initial version of spectrally calculated light transport in cycles?

What I can think of immediately are these:

  1. Similar noise per sample in a majority of scenes (benchmark scenes maybe) - this might favour one or the other depending on the specifics of the scene
  2. Only a minor performance hit for calculating samples on both CPU and GPU (not sure what can be considered acceptable here)
  3. No feature regression from master (bake, render passes, denoiser etc still work)
  4. Colour science is sound (colours come out as expected for simple examples such as white light on a diffuse coloured object)
  5. Code quality up to standard and code style following the styleguide. (Missed this one)

Are there any other requirements which would need to be met before such a patch would be considered to be merged?

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That seems like a good list, not sure there is anything to add, perhaps good code quality.

In terms of performance hit, I guess we can accept a few % slowdown. More samples needed due to noise increase will probably be a more significant factor though.

Depending on the numbers, we might want to keep support for rendering without spectral, where conversion between RGB and spectral would basically be a no-op.

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I took that one as a given :smile: I’m new to C and C++ so I might need a few rounds on cleaning things up, I suspect.

I’m hopeful we might actually get slightly better noise per sample in a majority of scenes, especially if I can get 4 wavelength hero wavelength sampling working alongside a wavelength PDF. There will of course be scenes which end up with more noise in spectral, but I will do what I can to minimise that.

This is pretty straight-forward to implement if we’re happy to accept spectral ‘primaries’ such as the ones from BT.2020, since we can just use the same 3 wavelengths for every sample. If we want to keep things exactly how they are now, that’s substantially harder, since it’ll mean a bunch of branching or duplication if I understand correctly.

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